1. Field of the Invention
The present invention concerns an improved device for detecting and indicating the presence or absence of molten slag in the taphole nozzle of a metallurgical vessel containing a combination of molten metal and molten slag, and particularly in a basic oxygen furnace (BOF) containing molten steel and molten slag.
2. State of the Art
To reduce the transfer of slag from the BOF into the ladle during tapping of the furnace, a means is required to identify the onset of slag at the end of a furnace tap. In most installations, slag is visually detected by furnace operating personnel. An improved method uses electromagnetic detection principles to distinguish the change in electrical conductivity between the conductive steel, and the non-conductive slag in the furnace taphole.
A system operating on this principle operates as follows: Two sensor coils are installed inside the taphole nozzle. One coil, the transmitter coil, establishes an electromagnetic field through the taphole area, and the other coil, the receiver coil, measures the strength of the field. When the taphole is filled with an electrically conductive material such as steel, the transmitter coil field will be partially shielded and accordingly a lower amplitude field will be sensed by the receiver coil. With non-conductive slag in the taphole, the field will not be as well shielded and an increased amplitude field will be sensed.
A major difficulty in implementing this approach is that the sensor coils must be installed inside the taphole nozzle, and must be rugged and well protected to operate through an entire furnace refractory campaign. Should the coils fail during a campaign, it would be difficult to repair or replace them without incurring a major delay in furnace operation. Installation of the coils is further complicated by the practice used to periodically replace the taphole refractory. Taphole refractory changes are typically made every 50 heats. To change a taphole, all the refractory materials inside the taphole nozzle are removed using a large, remotely operated hydraulic ram that follows the interface between the refractory and outer metallic nozzle shell casting to break up the old refractory. Sensor coils must therefore be installed in such a manner that they will not be damaged by the ram during such a taphole change. However, usually the sensor coils are located at the interface between the metallic nozzle shell and the refractory material, and therefore, the coils are broken when the hydraulic ram moves along the interface between the metallic nozzle shell and the refractory, to remove the refractory. There is a need therefore for an improved device for positioning inside a taphole nozzle of a metallurgical vessel, such that any removal of refractory from the refractory/metallic nozzle interface will avoid damage to the coils.
There is also a need for such device which can be quickly and conveniently removed for repair during the vessel campaign.